Many processes for obtaining copolymers of ethylene with 1-butene are already known. They operate either in gaseous phase without solvent or in liquid phase. These processes have many advantages as compared to the previous methods, such as high specific activity of the catalyst, power savings or simplified operating flowsheets. Practically it is often difficult to obtain 1-butene with the purity required to gain the benefit of all the advantages of these new processes.
As a matter of fact, 1-butene is mostly obtained by extraction from a cracking olefinic cut and unavoidably contains more or less subtantial amounts of the contaminents contained in said cut, for example isobutene, butadiene, 2-butene, butane and/or acetylenic hydrocarbons which either deactivate the catalyst or modify the molecular weight or consist merely of inert compounds increasing the weight of the recycled fractions.
One way of obtaining much purer 1-butene is to prepare it by selective dimerization of ethylene; the principle of such a process has been disclosed by K. ZIEGLER in U.S. Pat. No. 2,943,125 and has been considerably improved by using certain techniques such as disclosed in the European patent application No. EP-A-135 441 equivalent to U.S. Pat. No. 4,532,370, recommending as a catalyst alkyl titanate, trialkylaluminum and a modifier, with definite ratios and conditions of use.
This means provided a substantial improvement to the copolymerization process as a result of the use of the same starting material, ethylene, and of the purity of the produced 1-butene; but, on the other hand, it required the implementation, in addition to the copolymerization plant, of a second plant for a second process requiring its own catalyst and systems for removing inorganic compounds and for fractionation of ethylene, butene and unavoidable oligomers, and for recycling.
Now, it has been found that the titanium-containing catalyst already used for dimerizing ethylene to 1-butene and which is completely soluble in the reaction liquid medium is perfectly suitable for the preparation of a highly active catalyst in ethylene polymerization and ethylene copolymerization with 1-butene, in low temperature as well as in high temperature vapor phase processes, i.e. processes operating beyond 140.degree. C., at which temperatures the polymer becomes soluble in hydrocarbons or is in molten state.
As a result of this discovery, it is now possible to integrate the two processes to make them complementary of each other, without needing any catalyst removal, costly separations or recycling. The catalyst activity in each of the two steps is particularly well adapted to an integration since in the dimerization phase, it can produce from 20 to 100 kg of 1-butene per involved gram of titanium and, in the polymerization phase, it can produce 200 to 1000 kg of an ethylene-1-butene copolymer containing about 10% by weight of 1-butene per gram of titanium. Such an activity makes it unnecessary to remove the inorganic content of the polymer, all the catalyst elements being kept therein.